Support Option and similar enums as type of static variable with linkage attribute

compiler/rustc_codegen_gcc/src/consts.rs

@@ -8,13 +8,11 @@ use rustc_middle::mir::mono::MonoItem;
 use rustc_middle::ty::{self, Instance, Ty};
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::mir::interpret::{self, ConstAllocation, ErrorHandled, Scalar as InterpScalar, read_target_uint};
-use rustc_span::Span;
 use rustc_span::def_id::DefId;
 use rustc_target::abi::{self, Align, HasDataLayout, Primitive, Size, WrappingRange};
 
 use crate::base;
 use crate::context::CodegenCx;
-use crate::errors::LinkageConstOrMutType;
 use crate::type_of::LayoutGccExt;
 
 impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
@@ -239,12 +237,12 @@ impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
                     }
 
                     Node::ForeignItem(&hir::ForeignItem {
-                        span,
+                        span: _,
                         kind: hir::ForeignItemKind::Static(..),
                         ..
                     }) => {
                         let fn_attrs = self.tcx.codegen_fn_attrs(def_id);
-                        check_and_apply_linkage(&self, &fn_attrs, ty, sym, span)
+                        check_and_apply_linkage(&self, &fn_attrs, ty, sym)
                     }
 
                     item => bug!("get_static: expected static, found {:?}", item),
@@ -257,8 +255,7 @@ impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
                 //debug!("get_static: sym={} item_attr={:?}", sym, self.tcx.item_attrs(def_id));
 
                 let attrs = self.tcx.codegen_fn_attrs(def_id);
-                let span = self.tcx.def_span(def_id);
-                let global = check_and_apply_linkage(&self, &attrs, ty, sym, span);
+                let global = check_and_apply_linkage(&self, &attrs, ty, sym);
 
                 let needs_dll_storage_attr = false; // TODO(antoyo)
 
@@ -355,24 +352,12 @@ pub fn codegen_static_initializer<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, def_id
     Ok((const_alloc_to_gcc(cx, alloc), alloc))
 }
 
-fn check_and_apply_linkage<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, attrs: &CodegenFnAttrs, ty: Ty<'tcx>, sym: &str, span: Span) -> LValue<'gcc> {
+fn check_and_apply_linkage<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, attrs: &CodegenFnAttrs, ty: Ty<'tcx>, sym: &str) -> LValue<'gcc> {
     let is_tls = attrs.flags.contains(CodegenFnAttrFlags::THREAD_LOCAL);
     let llty = cx.layout_of(ty).gcc_type(cx, true);
-    if let Some(linkage) = attrs.linkage {
-        // If this is a static with a linkage specified, then we need to handle
-        // it a little specially. The typesystem prevents things like &T and
-        // extern "C" fn() from being non-null, so we can't just declare a
-        // static and call it a day. Some linkages (like weak) will make it such
-        // that the static actually has a null value.
-        let llty2 =
-            if let ty::RawPtr(ref mt) = ty.kind() {
-                cx.layout_of(mt.ty).gcc_type(cx, true)
-            }
-            else {
-                cx.sess().emit_fatal(LinkageConstOrMutType { span: span })
-            };
+    if let Some(linkage) = attrs.import_linkage {
         // Declare a symbol `foo` with the desired linkage.
-        let global1 = cx.declare_global_with_linkage(&sym, llty2, base::global_linkage_to_gcc(linkage));
+        let global1 = cx.declare_global_with_linkage(&sym, cx.type_i8(), base::global_linkage_to_gcc(linkage));
 
         // Declare an internal global `extern_with_linkage_foo` which
         // is initialized with the address of `foo`.  If `foo` is

compiler/rustc_codegen_gcc/src/errors.rs

@@ -211,13 +211,6 @@ pub(crate) struct InvalidMonomorphizationUnsupportedOperation<'a> {
     pub in_elem: Ty<'a>,
 }
 
-#[derive(Diagnostic)]
-#[diag(codegen_gcc_linkage_const_or_mut_type)]
-pub(crate) struct LinkageConstOrMutType {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(codegen_gcc_lto_not_supported)]
 pub(crate) struct LTONotSupported;

compiler/rustc_codegen_llvm/src/consts.rs

@@ -1,7 +1,7 @@
 use crate::base;
 use crate::common::{self, CodegenCx};
 use crate::debuginfo;
-use crate::errors::{InvalidMinimumAlignment, LinkageConstOrMutType, SymbolAlreadyDefined};
+use crate::errors::{InvalidMinimumAlignment, SymbolAlreadyDefined};
 use crate::llvm::{self, True};
 use crate::llvm_util;
 use crate::type_::Type;
@@ -162,22 +162,12 @@ fn check_and_apply_linkage<'ll, 'tcx>(
     def_id: DefId,
 ) -> &'ll Value {
     let llty = cx.layout_of(ty).llvm_type(cx);
-    if let Some(linkage) = attrs.linkage {
+    if let Some(linkage) = attrs.import_linkage {
         debug!("get_static: sym={} linkage={:?}", sym, linkage);
 
-        // If this is a static with a linkage specified, then we need to handle
-        // it a little specially. The typesystem prevents things like &T and
-        // extern "C" fn() from being non-null, so we can't just declare a
-        // static and call it a day. Some linkages (like weak) will make it such
-        // that the static actually has a null value.
-        let llty2 = if let ty::RawPtr(ref mt) = ty.kind() {
-            cx.layout_of(mt.ty).llvm_type(cx)
-        } else {
-            cx.sess().emit_fatal(LinkageConstOrMutType { span: cx.tcx.def_span(def_id) })
-        };
         unsafe {
             // Declare a symbol `foo` with the desired linkage.
-            let g1 = cx.declare_global(sym, llty2);
+            let g1 = cx.declare_global(sym, cx.type_i8());
             llvm::LLVMRustSetLinkage(g1, base::linkage_to_llvm(linkage));
 
             // Declare an internal global `extern_with_linkage_foo` which
@@ -195,7 +185,7 @@ fn check_and_apply_linkage<'ll, 'tcx>(
                 })
             });
             llvm::LLVMRustSetLinkage(g2, llvm::Linkage::InternalLinkage);
-            llvm::LLVMSetInitializer(g2, g1);
+            llvm::LLVMSetInitializer(g2, cx.const_ptrcast(g1, llty));
             g2
         }
     } else if cx.tcx.sess.target.arch == "x86" &&

compiler/rustc_codegen_llvm/src/errors.rs

@@ -61,13 +61,6 @@ pub(crate) struct InvalidMinimumAlignment {
     pub err: String,
 }
 
-#[derive(Diagnostic)]
-#[diag(codegen_llvm_linkage_const_or_mut_type)]
-pub(crate) struct LinkageConstOrMutType {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_sanitizer_memtag_requires_mte)]
 pub(crate) struct SanitizerMemtagRequiresMte;

compiler/rustc_error_codes/src/error_codes.rs

@@ -494,6 +494,7 @@ E0786: include_str!("./error_codes/E0786.md"),
 E0787: include_str!("./error_codes/E0787.md"),
 E0788: include_str!("./error_codes/E0788.md"),
 E0790: include_str!("./error_codes/E0790.md"),
+E0791: include_str!("./error_codes/E0791.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

compiler/rustc_error_codes/src/error_codes/E0791.md

@@ -0,0 +1,41 @@
+Static variables with the `#[linkage]` attribute within external blocks
+must have one of the following types, which are equivalent to a nullable
+pointer in C:
+
+* `*mut T` or `*const T`, where `T` may be any type.
+
+* An enumerator type with no `#[repr]` attribute and with two variants, where
+  one of the variants has no fields, and the other has a single field of one of
+  the following non-nullable types:
+  * Reference type
+  * Function pointer type
+
+  The variants can appear in either order.
+
+For example, the following declaration is invalid:
+
+```compile_fail,E0791
+#![feature(linkage)]
+
+extern "C" {
+    #[linkage = "extern_weak"]
+    static foo: i8;
+}
+```
+
+The following declarations are valid:
+
+```
+#![feature(linkage)]
+
+extern "C" {
+    #[linkage = "extern_weak"]
+    static foo: Option<unsafe extern "C" fn()>;
+
+    #[linkage = "extern_weak"]
+    static bar: Option<&'static i8>;
+
+    #[linkage = "extern_weak"]
+    static baz: *mut i8;
+}
+```

compiler/rustc_error_messages/locales/en-US/codegen_gcc.ftl

@@ -1,6 +1,3 @@
-codegen_gcc_linkage_const_or_mut_type =
-    must have type `*const T` or `*mut T` due to `#[linkage]` attribute
-
 codegen_gcc_unwinding_inline_asm =
     GCC backend does not support unwinding from inline asm
 

compiler/rustc_error_messages/locales/en-US/codegen_llvm.ftl

@@ -23,9 +23,6 @@ codegen_llvm_branch_protection_requires_aarch64 =
 codegen_llvm_invalid_minimum_alignment =
     invalid minimum global alignment: {$err}
 
-codegen_llvm_linkage_const_or_mut_type =
-    must have type `*const T` or `*mut T` due to `#[linkage]` attribute
-
 codegen_llvm_sanitizer_memtag_requires_mte =
     `-Zsanitizer=memtag` requires `-Ctarget-feature=+mte`
 

compiler/rustc_error_messages/locales/en-US/hir_analysis.ftl

@@ -113,3 +113,6 @@ hir_analysis_const_bound_for_non_const_trait =
 hir_analysis_self_in_impl_self =
     `Self` is not valid in the self type of an impl block
     .note = replace `Self` with a different type
+
+hir_analysis_linkage_type =
+    invalid type for variable with `#[linkage]` attribute

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1,4 +1,5 @@
 use crate::check::intrinsicck::InlineAsmCtxt;
+use crate::errors::LinkageType;
 
 use super::compare_method::check_type_bounds;
 use super::compare_method::{compare_impl_method, compare_ty_impl};
@@ -20,7 +21,7 @@ use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
-use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable};
+use rustc_middle::ty::{self, AdtDef, ParamEnv, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable};
 use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVENTIONS};
 use rustc_span::symbol::sym;
 use rustc_span::{self, Span};
@@ -478,6 +479,36 @@ fn check_opaque_meets_bounds<'tcx>(
     let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
 }
 
+fn is_enum_of_nonnullable_ptr<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    adt_def: AdtDef<'tcx>,
+    substs: SubstsRef<'tcx>,
+) -> bool {
+    if adt_def.repr().inhibit_enum_layout_opt() {
+        return false;
+    }
+
+    let [var_one, var_two] = &adt_def.variants().raw[..] else {
+        return false;
+    };
+    let (([], [field]) | ([field], [])) = (&var_one.fields[..], &var_two.fields[..]) else {
+        return false;
+    };
+    matches!(field.ty(tcx, substs).kind(), ty::FnPtr(..) | ty::Ref(..))
+}
+
+fn check_static_linkage<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) {
+    if tcx.codegen_fn_attrs(def_id).import_linkage.is_some() {
+        if match tcx.type_of(def_id).kind() {
+            ty::RawPtr(_) => false,
+            ty::Adt(adt_def, substs) => !is_enum_of_nonnullable_ptr(tcx, *adt_def, *substs),
+            _ => true,
+        } {
+            tcx.sess.emit_err(LinkageType { span: tcx.def_span(def_id) });
+        }
+    }
+}
+
 fn check_item_type<'tcx>(tcx: TyCtxt<'tcx>, id: hir::ItemId) {
     debug!(
         "check_item_type(it.def_id={:?}, it.name={})",
@@ -490,6 +521,7 @@ fn check_item_type<'tcx>(tcx: TyCtxt<'tcx>, id: hir::ItemId) {
             tcx.ensure().typeck(id.owner_id.def_id);
             maybe_check_static_with_link_section(tcx, id.owner_id.def_id);
             check_static_inhabited(tcx, id.owner_id.def_id);
+            check_static_linkage(tcx, id.owner_id.def_id);
         }
         DefKind::Const => {
             tcx.ensure().typeck(id.owner_id.def_id);
@@ -627,6 +659,7 @@ fn check_item_type<'tcx>(tcx: TyCtxt<'tcx>, id: hir::ItemId) {
                         }
                         hir::ForeignItemKind::Static(..) => {
                             check_static_inhabited(tcx, def_id);
+                            check_static_linkage(tcx, def_id);
                         }
                         _ => {}
                     }

compiler/rustc_hir_analysis/src/collect.rs

@@ -1814,7 +1814,12 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: DefId) -> CodegenFnAttrs {
             );
         } else if attr.has_name(sym::linkage) {
             if let Some(val) = attr.value_str() {
-                codegen_fn_attrs.linkage = Some(linkage_by_name(tcx, did, val.as_str()));
+                let linkage = Some(linkage_by_name(tcx, did, val.as_str()));
+                if tcx.is_foreign_item(did) {
+                    codegen_fn_attrs.import_linkage = linkage;
+                } else {
+                    codegen_fn_attrs.linkage = linkage;
+                }
             }
         } else if attr.has_name(sym::link_section) {
             if let Some(val) = attr.value_str() {

compiler/rustc_hir_analysis/src/errors.rs

@@ -285,3 +285,10 @@ pub struct SelfInImplSelf {
     #[note]
     pub note: (),
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_linkage_type, code = "E0791")]
+pub(crate) struct LinkageType {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -26,8 +26,10 @@ pub struct CodegenFnAttrs {
     /// The `#[target_feature(enable = "...")]` attribute and the enabled
     /// features (only enabled features are supported right now).
     pub target_features: Vec<Symbol>,
-    /// The `#[linkage = "..."]` attribute and the value we found.
+    /// The `#[linkage = "..."]` attribute on Rust-defined items and the value we found.
     pub linkage: Option<Linkage>,
+    /// The `#[linkage = "..."]` attribute on foreign items and the value we found.
+    pub import_linkage: Option<Linkage>,
     /// The `#[link_section = "..."]` attribute, or what executable section this
     /// should be placed in.
     pub link_section: Option<Symbol>,
@@ -113,6 +115,7 @@ impl CodegenFnAttrs {
             link_ordinal: None,
             target_features: vec![],
             linkage: None,
+            import_linkage: None,
             link_section: None,
             no_sanitize: SanitizerSet::empty(),
             instruction_set: None,

library/std/src/sys/unix/weak.rs

@@ -29,7 +29,21 @@ use crate::ptr;
 use crate::sync::atomic::{self, AtomicPtr, Ordering};
 
 // We can use true weak linkage on ELF targets.
-#[cfg(not(any(target_os = "macos", target_os = "ios")))]
+#[cfg(all(not(any(target_os = "macos", target_os = "ios")), not(bootstrap)))]
+pub(crate) macro weak {
+    (fn $name:ident($($t:ty),*) -> $ret:ty) => (
+        let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
+            extern "C" {
+                #[linkage = "extern_weak"]
+                static $name: Option<unsafe extern "C" fn($($t),*) -> $ret>;
+            }
+            #[allow(unused_unsafe)]
+            ExternWeak::new(unsafe { $name })
+        };
+    )
+}
+
+#[cfg(all(not(any(target_os = "macos", target_os = "ios")), bootstrap))]
 pub(crate) macro weak {
     (fn $name:ident($($t:ty),*) -> $ret:ty) => (
         let ref $name: ExternWeak<unsafe extern "C" fn($($t),*) -> $ret> = {
@@ -47,18 +61,39 @@ pub(crate) macro weak {
 #[cfg(any(target_os = "macos", target_os = "ios"))]
 pub(crate) use self::dlsym as weak;
 
+#[cfg(not(bootstrap))]
+pub(crate) struct ExternWeak<F: Copy> {
+    weak_ptr: Option<F>,
+}
+
+#[cfg(not(bootstrap))]
+impl<F: Copy> ExternWeak<F> {
+    #[inline]
+    pub(crate) fn new(weak_ptr: Option<F>) -> Self {
+        ExternWeak { weak_ptr }
+    }
+
+    #[inline]
+    pub(crate) fn get(&self) -> Option<F> {
+        self.weak_ptr
+    }
+}
+
+#[cfg(bootstrap)]
 pub(crate) struct ExternWeak<F> {
     weak_ptr: *const libc::c_void,
     _marker: PhantomData<F>,
 }
 
+#[cfg(bootstrap)]
 impl<F> ExternWeak<F> {
     #[inline]
     pub(crate) fn new(weak_ptr: *const libc::c_void) -> Self {
         ExternWeak { weak_ptr, _marker: PhantomData }
     }
 }
 
+#[cfg(bootstrap)]
 impl<F> ExternWeak<F> {
     #[inline]
     pub(crate) fn get(&self) -> Option<F> {

src/test/ui/feature-gates/feature-gate-linkage.rs

@@ -1,5 +1,5 @@
 extern "C" {
-    #[linkage = "extern_weak"] static foo: isize;
+    #[linkage = "extern_weak"] static foo: *mut isize;
     //~^ ERROR: the `linkage` attribute is experimental and not portable
 }